The influence of in vivo and in vitro ageing on the surface properties of normal human and rabbit erythrocytes will be studied. The relationship between density fractionated human red cells and their in vivo age will be established. Several density fractionation techniques for the red cells will be compared including phthalate ester mixtures, albumin, ficoll and centrifugation in the absence of additives. In vivo 59Fe cohort labelled normal red cell populations will be fractionated at various periods through their life-span. The fractions of different density will be characterized in order to establish the relative adequacy of the fractionation techniques for preparing cells of different ages as well as to identify useful indicators for red cell age (e.g. activity of various enzymes, level of membrane-bound NANA, cell volume, etc.). Methods will also be refined for the enzymatic release and subsequent analysis of total membrane-bound sialic acid. Red cells from human blood stored under standard blood banking conditions will be examined in order to identify useful indicators for red cell age in vitro. Properties to be examined will include surface charge, levels of activity of surface and intra red cell enzymes, membrane-associated sialic acid, cell size and cell density. Animal model studies will be carried out using rabbits. Basic characterization of fractionated rabbit red cells and experiments with the same purpose as those described for human red cells will be initiated.